The present invention relates to a rotor structure for a rotary wing aircraft. More specifically, the invention relates to a rotor having an even number of rotor blades secured to the rotor head in pairs so that the rotor blades of a pair are arranged diametrically opposite each other. The members of a pair are interconnected by a common spar extending from one blade root of a pair to the other blade root of a pair, as an integral structure. The spar is sufficiently yielding to torsion loads resulting from blade angle movements. In other words, the spar is torsion soft. Similarly, the spar is sufficiently yielding against bending loads resulting from blade flapping movements and from blade lead-lag movements. German Patent Publication (DE-OS) 2,755,557 discloses a rotor without any defined flapping hinges and without any lead-lag hinges for the rotor blades. In such a structure the bending moments resulting from the blade flapping and from the blade lead-lag movements are transmitted to the rotor head through the spar member. Relatively soft zones of the spar contribute to relieving the rotor blades of these moments. In such rotor structures it cannot be avoided that the blade angle adjustments cause a twisting of the spar member because the latter necessarily has a different height and width due to stiffness considerations. As a result, such twisting causes a change in the bending stiffness of the spar member in the zone that is twisted. Such zone extends from the respective blade wing to the connecting point of the spar to the rotor head. As a result, in this type of rotor having a common spar member for each pair of rotor blades, the blade angle adjustment movements affect the blade flapping movements and the blade lead-lag movements in such a manner that the ficticious flapping hinge spacing and the ficticious lead-lag hinge spacing from the rotor head vary in response to the blade angle adjustment. Accordingly, this type of prior art rotor has the disadvantage of an undefined vibration characteristic. Strong vibrations may occur with the result that the rotor and the aircraft cabin or fuselage are subjected to high loads.
German Patent Publication (DE-OS) 2,701,519 describes a rotor structure in which the wing blades are also interconnected in pairs by spar members made of fiber reinforced synthetic material whereby the fiber strands extend in the longitudinal direction of the blades and spar members.